Add extra error messaging

[itwasmattgregg]

Add a little more explanation to netlify-lambda build error.

This was my first pass at some additional error messaging.
For me it was confusing seeing this error and not knowing I needed to have a separate src and dist directory and that the netlify.toml file only needed to be told about the dist. I assumed netlify-lambda did the build process for you and only needed the src directory.

[swyxio]

heheh. ok. tbh i'm not too fussed about it, its literally the one concept people need to get about netlify-lambda and i've spent a lot of effort documenting in README and in that error you edited. but thanks